The preparation of silica is usually done by either of two methods. According to one method, silica is prepared by converting a silica-hydrogel precipitate, that is then washed and dried. Products prepared in accordance with a method of this type are usually called silica gels and are mainly used as adsorbents and as catalyst supports. In order to be able to meet some of the various criteria for catalyst supports, a great many different embodiments of this type of process have been proposed and described, e.g., in Netherlands Patent Applications Nos. 69.11999 and 69.12002, German Patent Application 2,411,735, Canadian Patent Specification No. 967,936, and U.S. Pat. Nos. 2,700,061; 2,731,326; 2,763,533; 2,765,242; 2,785,051; 3,428,425; 3,433,593 and 3,453,077.
For some other uses, such as fillers, reinforcing agents, thickeners, and the like, silica is prepared by flame hydrolysis of silicon-halogen compounds, i.e., conversion of these compounds with a combustible hydrogen-containing gas.
The particle size of silica prepared by flame hydrolysis is considerably smaller than that of the silica gels. A silica prepared by flame hydrolysis consists of primary particles ranging from a few millimicrons to at most a few dozens millimicrons. These primary particles usually agglomerate into bigger, so-called secondary particles, the particle size of which usually range from about 1 to about 10 microns.
The particle size of the silica gels can readily be controlled, and these gels can also be prepared in the form of granules or small lumps.
According to a well-known method for preparing polyolefins, notably polythene, an .alpha.-olefin, e.g., ethene, is polymerized with the aid of catalysts based on chromium dioxide on a support such as, silica. Catalysts of this type are prepared by impregnating the support with a solution of chromium oxide, a compound that can be converted into chromium oxide, or a composition containing chromium oxide, drying the impregnated support and activating the catalyst composition by heating it at temperatures between about 400.degree. C. and about 1000.degree. C. in a non-reducing atmosphere such as, an inert atmosphere like nitrogen or carbon monoxide, or in an oxidizing atmosphere preferably air. Catalysts of this type are sometimes referred to as Phillips-type catalysts. These supported chromium-oxide catalysts may be used as such, but organometallic compounds may also be added.
The most practical method of activating the catalyst composition is by heating in a fluidized bed. Such a method, however, can be used only if the silica support has a given minimum particle size, as otherwise a considerable part of the supported catalyst will be blown out of the fluidized bed causing major dust problems. For this reason, silica prepared by flame hydrolysis which has a small particle size is not desirable as a catalyst support. In order to be able to heat the supported catalyst in a fluidized bed at temperatures between about 400.degree. and about 1000.degree. C., the particles must have not only a minimum particle size, but also must have sufficient strength to withstand the strong abrasive forces in the fluidized bed reactor. Otherwise, strong abrasion and pulverization will occur in the fluidized bed forming large fractions of fine particles which cause dust problems and which are not suitable as particles of supported catalyst.
It has also been discovered that the support must meet certain other requirements in order to produce a catalyst which obtains high yields of polyolefins with good product properties. One of these requirements is that the silica support must have a given porosity, which must be retained as much as possible when the support is heated. In this regard, the content of impurities, notably the sodium content, was discovered to be of critical importance. When this content is high, the pore volume decreases when the silica is heated, in some cases the pore volume has decreased to very low values of no more than a few tenths of cm.sup.3 /g. This decrease in pore volume has an unfavorable effect on the activity of the catalyst. Even if the pore volume decrease is comparatively small, if there is too high of a sodium content, the activity of chromium oxide and similar catalysts will still be low. The cause of this decrease in activity of the catalyst is not yet clear. One suggestion in that crystallization phenomena play a part, but this is still an open question.
A drawback of many Phillips-type catalysts is that the melt index of the polyethylene to be produced can be controlled effectively only by means of the polymerization reactor temperature. Because the sensitivity of these catalysts to hydrogen as a molecularweight regulator is slight, large amounts of hydrogen are generally used to regulate the molecular weight.
For producing polyethylene with comparatively high melt indices, the solution process is suitable, i.e., polymerization is effected at temperatures of at least about 110.degree. C. and a solution of polyethylene in the solvent used, e.g., gasoline, is obtained. The problem with the solution process is that it is more expensive than the suspension process. This is because the polymerization in the suspension process is effected at a lower temperature, generally about 65.degree. C. to about 85.degree. C.
Phillips-type catalysts, and notably supports for these catalysts now exist for the preparation of polyethylene with comparatively high melt indices by polymerization in a suspension process. However, the preparation of supports for catalysts of this type has been cumbersome and time-consuming. Sodium-silicate solution is used as a starting material and silica is made of precipitate from it by means of an acid, usually sulphuric acid. The supports must be thoroughly free of sodium, therefore, washing for prolonged periods is required to obtain the desired low sodium concentrations. As a result of this cumbersome process, the cost of making these supports has been high. Up until now, other suggestions for supports or catalysts for the preparation of polyethylene with comparatively high melt indices of polyethylene with comparatively high melt indices in a suspension process have so far given few, if any, results.
It has now been discovered that chromium-oxide catalysts on a silica support prepared according to the present invention are highly sensitive to hydrogen and that they allow the preparation of a polyethylene of any desired melt index by the suspension process.
The catalyst on support, and hence the support, must have a given minimum particle size, because in addition to reasons discussed above, the particle size of the support also affects the particle size of the polymer. During transport, trans-shipment and processing, a fine polymer with small particle sizes causes dust problems that become more serious as the particle sizes of the polymer become smaller. Therefore, the support must have an average particle size of at least about 10.mu., preferably, at least about 40.mu., and even more preferably, at least about 80.mu..
The powder properties of the polymer are such that it is desirable that the particle-size distribution of the polymer be in a narrow range, i.e., the particle size distribution factor according to Rosin-Rammler must be at least 2. In addition to the dust problem discussed above, the particle size distribution also affect the flowing and handling properties of the material, i.e., the ability of being able to move the material from container to container during shipment and further processing. In order to obtain polymer powders of this type, the particle size distribution factor of the support must also be at least 2. The determination of the average particle size and particle-size distribution can be done by a screen analysis. The results of the screen analysis can then be plotted in a double logarithmic-logarithmic Rosin-Rammler diagram. The point on the curve corresponding to a screen residue of 36.8% indicates the average particle size. The gradient of the curve is a measure of the width of the particle-size distribution. The steeper the gradient, the greater the uniformity factor and the narrower the particle-size distribution. Particles which are too coarse are also not too desirable. The average particle size is preferably at most about 250.mu., and, even more preferable, at most about 200.mu., at a particle size distribution factor of at least 2 and, preferably at least 3.